This invention relates to an improved digitizing scanner, and more particularly to a scanner for reading and storing graphical and textual image data from transparent and translucent sheets such as developed X-ray film.
Electro-optical digitizing scanners are commonly employed as peripheral devices linked with microcomputers and other data processing and storage devices. Scanners enable graphical and text data to be accurately converted into stored digital data for further processing and interpretation by, for example, a microcomputer. Scanners are adapted to read data from a variety of media and formats. Opaque and transparent sheets are two common forms of scanned media.
An image on a sheet is defined by light areas (xe2x80x9chighlightsxe2x80x9d) and dark areas (xe2x80x9cshadowsxe2x80x9d). To convert the light and dark areas into corresponding image data, the scanner typically illuminates the sheet with a light source. In one form of scanner, a camera assembly moves along the length of the sheet. In another, the sheet moves relative to a stationary camera. As the sheet moves relative to the camera, the camera xe2x80x9cscansxe2x80x9d the width of the illuminated image, converting the scanned portion of the image into a data signal. This scanned image is said to be xe2x80x9cdigitizedxe2x80x9d in that the image is converted into a data file stored in a digital format with information representative of discrete segments or xe2x80x9cpixels.xe2x80x9d The data in the file includes instructions on how to assemble the individual pixels into a cohesive two-dimensional image that reflects the original scanned image. The data file also includes information on the intensity value for each pixel and its color, if applicable, or grayscale shade.
A common form of camera assembly for use in a digitizing scanner is the solid-state CCD camera, which contains a linear array of photosensitive picture elements, often termed xe2x80x9cpixels.xe2x80x9d Each pixel element receives light in its local area. The pixel generates an intensity-based signal depending upon how much light it receives. The aggregate signal of all the pixel elements is a representation of a widthwise xe2x80x9clinexe2x80x9d of the image.
Generally, the CCD pixel array only scans a single line that is several thousand pixels wide in the fast scan direction but that has a height of only one pixel in the slow scan direction. The array is typically wide enough to scan the entire image width at once. Because an entire line is generally viewed at once, this is known as the xe2x80x9cfast scanxe2x80x9d direction; since the delay is only in downloading the signal from the CCD to the data processor. Conversely, the direction of movement of the camera/image is known as the xe2x80x9cslow-scanxe2x80x9d direction. In summary, images are scanned in a xe2x80x9cline-by-linexe2x80x9d manner in which the image moves in the slow scan direction relative the camera""s fast scan field of view. As the image passes through the field of view, a succession of scanned width-lines of the image are converted into image data, and the CCD element generates a continuous signal representative of the intensity of each pixel in the line.
Scanners used for scanning opaque sheets must illuminate the image by reflecting illumination light off the surface of the sheet from the same side as the camera. Conversely, when transparent or translucent sheets are scanned, the image is illuminated from the opposite side of the sheet from the camera, allowing the light to pass through the image to the camera. In this manner, the image attenuates the light as it is transmitted through the sheet to the camera.
CCD elements are generally smaller in width than the scanner""s total scan width. A focusing lens is employed to focus illumination light from the scanned image onto the narrower viewing area of the CCD. The focused image will generally exhibit a degradation in the field of view at the far edges of the width (e.g. a loss of exposure). This loss of exposure occurs because the amount of light entering a lens tends to decrease at the edges of the field of view according to the Cos4 characteristic of lenses. It is often desirable to increase the light near the edges of the camera""s field of view to compensate for this effect. However, most illuminators comprise only one or two discrete light sources, such as a long fluorescent bulb. The intensity of such a bulb is not generally controllable along its length. In fact the bulb may exhibit variability in light output along its length, presenting a different level of intensity to different pixels in the array. This problem becomes exacerbated as the bulb ages. In addition, the pixels of the CCD camera may exhibit different responses to the same intensity of light. The CCD pixels can be calibrated to account for most variations, but it is desirable to have the capability of changing the profile of light presented to the various pixels. In general compensation for an uneven light profile is difficult using a single illumination bulb.
Scanners derive a large quantity of information from a single sheet containing an image. When a sizable number of images are stored for long-term use, superfluous data related to edges and margins can become a concern. Substantial computing resources in both time and storage capacity can be devoted to unneeded data. In particular, images substantially narrower than the maximum field of view of the scanner are often scanned as if the full width (in fast scan direction) of the scanner is employed. It is desirable, therefore, to accurately gauge the size of the needed data range, and only scan the image within the needed range in both the fast scan and slow scan directions. In the past this has been accomplished primarily by manually inputting the size of the sheet to be scanned. Alternatively, movable edge guides can be linked to a size sensor that inputs the relative width of the input sheets. An electromechanical/optical length sensor starts and ends the scanning process as the front and rear edges of the sheet pass through the scanner. However, these techniques still require accurate registration of input sheets and do not determine the size of the margins.
The scanning of translucent sheets is desirable in the medical field, and presents particular challenges. In particular, there is a need to digitally store and reproduce diagnostic radiological films, commonly termed xe2x80x9cX-rays.xe2x80x9d Most patient X-ray films, in fact, are produced in a xe2x80x9cseriesxe2x80x9d that can consist of six or more individual, interrelated X-rays. Hundreds, or even thousands, of X-ray films are produced daily by a large hospital. By electronically storing and indexing radiological images, they can be made available indefinitely without taking up valuable physical storage space. In addition, various specialized graphical processes and image enhancement techniques can be used in connection with stored X-ray images. Furthermore, scanned radiological data can be easily transmitted to practitioners at remote locations via electronic mail or facsimile. In all, the ability to accurately and reliably scan developed X-ray film images provides an important diagnostic tool for medical practitioners.
The scanning of developed X-ray film presents some particular challenges. X-rays tend to exhibit a large area of shadows with both abrupt transitions, and more subtle dark, clouded areas. Hence, the CCD element intermittently must operate at a low output level throughout the scanning process. Low light intensity causes the CCD element to transmit a corresponding low output signal. Electronic noise is accentuated at this low output level, causing inaccuracies in the scanned image data. Incandescent and fluorescent light sources often have short life spans that may render them unsuitable for a large volume radiological scanner. Alternatively solid-state illumination devices, such as light emitting diodes (LEDs) must be used in large arrays. While they are energy-efficient, long-lived, and consistent over their service life, they may have wide variability in output intensityxe2x80x94even LEDs in the same production batch. Thus the light intensity pattern presented by an unadjusted array of LEDs can exhibit substantial, undesirable variation in intensity across the scan width.
An illuminator that is larger in width that the image can cause imaging problems. Scattered, stray light from the outer edges of the illuminator, beyond the scanned width of the image, can cause distortion and refraction patterns in the optics of the camera assembly that degrade the scanned image. The width of projection of most light sources, such as elongated fluorescent bulbs, cannot be easily or reliable varied.
Medical X-ray film images are usually scanned at approximately 150 pixels per inch (PPI) resolution, since this value enables a 14-inch image to be displayed on a standard 2,000-pixel-wide monitor. This resolution is generally considered sufficient for radiological data storage and reproduction purposes. The native resolution of many currently available CCD camera elements is approximately 8,000-12,000 CCD picture elements. Divided over a 14-inch image this number of picture elements can provide native resolutions of at least four times the number of pixels called for. It is desirable to derive image data at the lowest needed resolution to reduce scanning time and storage requirements. Lower resolution is also desirable when transmitting data over low-speed transmission lines to save time. An efficient technique for changing the resolution of the system is desired.
Some circumstances may warrant the inclusion of specific image data details at a higher resolution. These details are regions of particular interest on an overall xe2x80x9cparentxe2x80x9d image. It is desirable to provide a technique for producing higher resolution image files of regions of interest, and electronically associating these high-resolution detail files with the overall xe2x80x9cparentxe2x80x9d image.
While the individual pixels of currently available CCD camera elements exhibit relatively consistent pixel-to-pixel output, there is still signal variability between individual pixels in an array. In particular, the signal for a dark image (the xe2x80x9cdark currentxe2x80x9d signal) can vary significantly from pixel to pixel. An adjustment function is used to vary each pixel""s output signal so that it attains a desired uniform value. In particular, look-up tables based upon predefined smoothing functions are often used to provide a positive or negative adjustment bias to individual CCD pixels. The gain exhibited by each individual CCD pixel is also adjusted by deriving the change in output signal for a standard dark image and a standard light image. The output signals of each of the thousands of pixels in the array must be adjusted with an individual set of bias and gain adjustment factors. This adjustment procedure consumes substantial amounts of processing time and resources as the linear output of each pixel is summed with an appropriate positive or negative bias factor to provide each pixel with an approximately equal dark current response. It is often desirable to deliver a final signal from a CCD pixel in a logarithmic, or another non-linear converted form. If the final non-linear signal is to be sampled to provide the basis for adjusting the bias at the linear input stage, then the device must have an accurate representation of the function being used to convert the linear signal into the final, non-linear signal. With foreknowledge of the type of non-linear output to be expected for a given linear input, the function can determine the mount of bias needed at the linear stage to generate a proper shift in the output at the non-linear stage. In other words, if the non-linear output is off by x, then the function knows that a deviation of Log(x) has occurred at the linear stage and this value is a correction factor to the input.
However, preprogrammed tables of bias correction factors do not always accurately predict the real response of a system. Likewise many signal processing functions cannot be easily characterized. Using a function or a preprogrammed table of expected correction factors to effect CCD pixel bias calibration can result in inaccurate data and can expend substantial time and computing resources. This inaccuracy can be encountered even in linear signal conversion circuits.
Notwithstanding the foregoing, reliability, repeatability of results and ease of use remain a significant concern in any medical device. In particular, a medical digitizing scanner must meet certain guidelines promulgated by the US Food and Drug Administration and other regulatory agencies in the United States and abroad.
In view of the foregoing disadvantages of the prior art, it is an object of this invention to provide a digitizing scanner, particularly applicable to translucent sheets such as X-ray film having an improved illumination system and camera arrangement that produces highly accurate and consistent scanned image data. The illumination system should be self-calibrating, have a long service life and should compensate for optical and light source inconsistencies. The camera arrangement should operate efficiently at a desired resolution, should minimize distortion, exhibit a high degree of optical precision and should include adequate noise suppression capabilities for enhancing the quality of scanned images. The camera element should be readily calibrated, particularly in the logarithmic output signal domain. In addition, the size of the image to be scanned should be accurately determined and located automatically. The illuminator width should be readily adjustable to fit the size of the image.
The digitizing scanner according to this invention overcomes the disadvantages of the prior art by providing a plurality of improved components and functions. In a preferred embodiment, the digitizing scanner is generally arranged so that an illuminator transmits light through a transparent or translucent sheet into a stationary camera assembly. The image on each sheet passes through the field of view of a linear CCD camera assembly as sheets are driven lengthwise in the xe2x80x9cslow scanxe2x80x9d direction by a feed roller assembly. The CCD captures a succession of lines of the image. Each line is oriented widthwise, in the xe2x80x9cfast scanxe2x80x9d direction. Images are transmitted as an image signal to the scanner""s central processing unit (CPU) and to a microcomputer or other data processing/storage device as image file data. The sheets can comprise developed X-ray film having black and white radiological images thereon. The CCD can transmit information according to a corresponding black and white xe2x80x9cgrayscale.xe2x80x9d
According to one embodiment, the scanner includes an improved illuminator for illuminating an image. The illuminator consists of a linear array of individually controllable light emitting diodes (LEDs). The driving current for each LED is varied during a calibration procedure in which the output light intensity of each LED is independently measured by the scanner""s camera, and the driving current is adjusted in a series of adjustment cycles, or xe2x80x9cpassesxe2x80x9d to provide a predetermined consistent light output across the array. The light output pattern naturally adjusts for inherent optical and camera inconsistencies, since the output is varied based upon the pattern actually viewed by the camera. The LED array can include a photosensitive sensor that measures the light output of one LED to derive a reference light intensity. The other LEDs in the array are calibrated based upon this reference. A coarse intensity adjustment can also be employed before each LED is individually adjusted. Adjustment typically occurs in increments, varying the LEDs driving current as a product of the old current times the ratio of the average array illumination level versus the LED""s illumination level.
In another embodiment, a housing for the LED array can include a pair of tapered walls that enclose part of each LED""s bulb. The walls taper to a narrower opening adjacent a translucent diffuser window. The illumination light projected by the LEDs exits the diffuser window in a highly diffuse form.
In another embodiment, a secondary illuminator is provided adjacent the same face of the sheet as the camera assembly. The illuminator can comprise a variety of acceptable light sources arranged to project a reflected light onto a predetermined section of the sheet, typically in a margin. The predetermined section includes an opaque bar code strip or another identifier. The CPU can include instructions for reading and interpreting the strip or identifier, and can control the procedure for reading the strip at predetermined times.
In another embodiment, the camera assembly comprises an enclosure having a sealing window oriented in the widthwise direction for receiving light transmitted from the image. The window allows light to strike a series of reflectors that define an optical path. The optical path terminates at a focusing lens and the CCD camera element. A transparent covering window is positioned between the focusing lens and the CCD camera element, adjacent the CCD camera element. The sealing window is oriented at a non-perpendicular angle to a plane passing perpendicularly through the optical path to divert stray light out of the optical path. In one embodiment the angle is set preferably at 7xc2x0-15xc2x0. However, any angle that enables diversion of stray light without unduly compromising the optical performance of the camera assembly is acceptable. The camera and the covering window, as a unit are tilted in the housing at an angle preferably between 7xc2x0 and 15xc2x0 relative to a plane passing perpendicularly through the optical path. The reflectors can be mounted on a rigid frame member on respective adjustable mounts.
In another embodiment, the output image signal from the CCD element can be processed dynamically to reduce noise in the low-intensity (shadow) signal range. A two stage logarithmic amplifier is employed to amplify the signal by 100 dB in two 50 dB stages. A variable low-pass filter reduces the bandwidth of the signal between the two stages according to predetermined criteria. Specifically, a control amplifier controls the filter""s maximum allowable signal bandwidth based upon the current value of the output image signal. For low-intensity output signal values below a predetermined lower limit, a minimum allowable bandwidth is selected. The allowable bandwidth increases to a maximum value wherein a predetermined upper limit is reached. This upper limit is at the upper end of the low-intensity output signal value range. The filtered output signal of the filter is passed through the second stage of the logarithmic amplifier and the output of the second stage amplifier is summed with the output of the first stage logarithmic amplifier to produce a 100 dB filtered logarithmic output signal. This signal is converted into useable digital and linear form by appropriate converters.
In another embodiment, resolution of the CCD camera element is reduced from a higher resolution by averaging the values of adjacent fast scan pixels in the fast scan direction and deriving a single pixel intensity for the entire grouping. Pixel intensity values are preferably combined in adjacent pixel groupings of 2, 4 or 8. The resulting summed intensity values are averaged in the binary domain by shifting the sum by 1, 2 or 3 bits, respectively. Averaging of pixel values in the slow scan direction is accomplished by varying the scan speed of the image to present a plurality of lines to the CCD camera array in a given scan cycle. In a preferred embodiment, the scan speed is varied by controlling the operating speed of the feed roller drive motor. The CCD camera element samples lines at a fixed rate. By increasing the scan speed, a larger area in the slow scan direction is presented to the camera during each sample cycle. The area scanned is read by the CCD camera element as an average intensity signal for each CCD pixel. Preferably, 2, 4 or 8 lines are averaged, resulting in an average line signal that represents a line of pixel intensity values for the entire grouping of lines. The average pixel values derived from either, or both, fast scan and slow scan pixel averaging are stored as an image data file according to the new, reduced resolution.
In another embodiment, the size of a sheet fed into the scanner is automatically determined, and the amount of data taken and stored by the scanner is adjusted using the intensity readings of the camera element. The sheet is fed by the rollers into the field of view of the camera assembly. The camera begins scanning before the sheet arrives at its field of view. The intensity transition between the free space before the edge of the sheet and the attenuated intensity as the sheet passes into the field of view is identified by the CPU as the lead edge of the sheet. The CPU maps the location of the lead edge to the feed motor""s position by counting steps or reading another movement sensor signal operatively connected to the motor. The widthwise edges of the sheet are then determined by locating intensity transitions on either side of the sheet along the fast scan direction. The location of the side edges can be mapped at predetermined intervals relative to the motor""s position or the location can be mapped continuously. The CPU continuously polls for a second intensity transition at the tail edge in the slow scan direction. The second transition, when identified by the CPU, is mapped relative to the motor""s location by the CPU and the sheet is reversed by the rollers until the top margin is again upstream of the camera assembly""s field of view. The sheet is then fed by the rollers through the scanner again. Only data falling substantially within the mapped boundaries of the sheet, based upon the current position of the motor, are acquired and stored for further processing.
In another embodiment, the illumination assembly is adjustable to deactivate selected light sources having respective centers of projection that fall outside of the widthwise edges of the sheet, or another set of widthwise limits. The programmable current sources for selected light sources are instructed by the CPU to assume a minimum current or xe2x80x9coffxe2x80x9d setting. The deactivated light sources can be selected based upon their known physical locations along the width of a sheet. Selection can occur based upon manually input width measurements or based upon automatic size sensing functions, such as the procedures described above.
In another embodiment, the image is annotated with stored identifiers that link the image to a higher resolution file having image data related to a specific area of interest on the main image. The linking can be made according to the ANSI-DICOM-3 standard. According to this standard the main image is stored in a predetermined format. A high-resolution subfile is created by rescanning a particular region of the sheet containing the main image. This subfile is also stored in the predetermined format, and the two files are linked for subsequent display and data transmission. The use of a smaller high-resolution linked file saves valuable storage capacity and data handling time, particularly during data transfer to remote sites. Sheets remain in the feed rollers of the scanner until all scans have been accomplished, selectively driving the sheet in reverse, and forward again until all scanning operations have been completed.
In another embodiment, a method for bias calibration of CCD pixels in the camera array that enables efficient adjustment of the bias of individual CCD pixels in the linear mode based upon a sensed output of each of the pixels in the logarithmic domain is provided. A group of pixels from the overall array is exposed to a substantial absence of light representing a maximum dark intensity image. This is accomplished by deactivating the illumination assembly. The linear output of each of the pixels is amplified using a signal converter, such as a logarithmic amplifier, and more particularly, the two-stage amplifier employed in the above-described filtering circuit. Each CCD pixel in the group is assigned a specific bias value that is summed with the respective pixel""s output to the dark intensity to create an incremental ramp of bias-adjusted linear inputs to the logarithmic amplifier that are amplified into a set of logarithmic system response values at the output of the logarithmic amplifier. The ramp of individual bias values is stepped incrementally from a minimum negative bias value to a maximum positive bias value. The minimum negative value and the maximum positive bias value are of equal magnitude and opposite sign of voltage/current according to a preferred embodiment. The bias values between the minimum and maximum are equal, increasing increments of voltage/current. The logarithmic amplifier is precalibrated to generate a minimum, negative system response output in the logarithmic domain when the minimum bias value is input and a maximum, positive system response output in the logarithmic domain when the maximum bias value is input. The negative and positive system response output values are, likewise, equal in magnitude, and opposite in sign according to a preferred embodiment. Each logarithmic domain system response is mapped to the input bias that produced the response. The mapping process results in a table or xe2x80x9ccurvexe2x80x9d of logarithmic system response versus input linear bias. The approximate middle of the curve represents a desired system response of 0 for the CCD array. A given bias value on the ramp (approximately half-way between the minimum and maximum value) produces a baseline, 0-system response. The values for data points in the curve can be represented by numerical digital integers that are translated using appropriate digital/analog and analog/digital converters. The curve of system response versus bias is manipulated through inversion and translation to derive another curve (in look-up table form) of bias adjustment factors for a respective set of deviation values from the desired base response for a pixel at a dark current output. The response of each pixel in the array is then measured in the logarithmic domain. The curve is queried to assign the appropriate linear bias adjustment factor to each pixel based upon its logarithmic output. The bias adjustment factor assigned to each pixel is mapped to that pixel and applied to the linear output of that pixel each time it transmits an intensity signal. According to a preferred embodiment, eight bias and associated system response data points can be summed to generate a single look-up table point. Deviations from the base value that fall between the averaged points can be derived through linear interpolation. By assigning a ramped bias to each pixel in a group that spans approximately one decade of logarithmic output, a bias correction factor table can be developed in a single 8-millisecond scan cycle. Curve-smoothing and point-averaging functions are employed to ensure that variations in the output signals of individual pixels do not unacceptably disrupt the continuity of the response curve.
It is expressly contemplated that any of the above-described embodiments can be employed in conjunction with one or more of the other above-described embodiments in the digitizing scanner according to this invention.